CAIRO — Having won the presidential vote, Egypt’s main Islamist group, the Muslim Brotherhood, squared off on Monday against the nation’s military rulers, who showed few signs of ceding ultimate authority to the longtime outlaw group.

Flush with victory, the Brotherhood sought to use the electoral success of its candidate, Mohamed Morsi, as the linchpin of a three-pronged assault on the ruling military council, which had prepared for the possibility of an Islamic victory by dissolving the Brotherhood-dominated Parliament and seizing all legislative power.

The Brotherhood leaders vowed that they would simultaneously engage the generals in political negotiations, challenge them in the courts and threaten them with unrest in the streets.

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Supporters of Mr. Morsi celebrated, but election officials will not formally confirm the presidential results until later in the week.Credit
Daniel Berehulak/Getty Images

“They neither have legislative power or presidential power,” said Jihad el-Haddad, a Brotherhood spokesman. “We don’t need to hear any excuses from them. If they want to hand over power, they can give back the Parliament and undo what they have done.”

Although the winner of the presidential race will not be confirmed officially until later this week, the Brotherhood and the military council both quickly moved beyond the ballot box to make their case to a public weary from a revolution and political infighting.

The ruling generals sought for the first time to sell the public on the decision to dissolve the Brotherhood-led Parliament on the eve of the vote. In a nearly two-hour news conference that was edited before it was televised, two members of the military council insisted that they regretted dissolving Parliament, but that they had been forced by a court ruling from judges appointed by former President Hosni Mubarak.

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Brotherhood supporters called the apparent victory by the Islamist candidate, Mr. Morsi, a rebuke to the military’s power grab.Credit
Ahmed Jadallah/Reuters

And although they have now issued an interim Constitution keeping legislative and much of the executive power for themselves — and even said later Monday that they would appoint a general to run the new president’s staff — the generals promised to hold a “grand celebration” when they turned over power as promised at the end of the month.

“We gave the People’s Assembly its complete powers,” Maj. Gen. Mohammed el-Assar added, glossing over the council’s dissolution of the body, “and we will give the president of the republic his complete powers.”

But as much as this was a new battle for Egypt’s future, with an Islamist elected as head of state for the first time in the Arab world, historians said it fit an all-too-familiar pattern. The combination of public protests and back-room negotiations followed a decades-old dance between the Brotherhood and the Egyptian military — each in its own way a conservative institution that has tended to reach for accommodation rather than all-out war.

But in the current contest over Egypt’s future, the grounds for compromise were unclear.

The generals appeared to want to “upgrade Mubarak’s authoritarianism by adding free and fair elections,” while preserving broad powers for themselves over matters including defense, national security and perhaps some broad economic issues, said Mona el-Ghobashy, an Egyptian political scientist at Barnard, who is now in Cairo.

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A woman walks in front of a wall near Tahrir Square in Cairo on Monday that reads "president for tomorrow."Credit
Mohamed Abd El Ghany/Reuters

“But the brothers want to be part of the power structure,” she said. “They are not going to accept their wonderful electoral institutions that basically are just hamstrung and get all the blame for things. And this issue is going to be at the heart of Egyptian politics for a long time.”

In their news conference, the generals acknowledged they would have a monopoly on all lawmaking powers as well as control of the national budget. But they said that the new president — they did not name Mr. Morsi — would retain a veto over any new laws and could name the prime minister as well as other cabinet officials.

But the military declined to back away from the core of an interim charter they issued late Sunday night — just as the polls closed — that removed the military and the defense minister from presidential authority and oversight. They also defended their imposition of martial law, granting the military the power to detain civilians for trial in military courts.

Egyptian state media reported later on Monday that the generals had continued to expand their power at the expense of the president. They named one of their own, Gen. Abdel Momen Abdel Baseer, to be the new president’s chief of staff, directing much of the few financial and personnel affairs ostensibly left under the president’s control. And they revived a special national defense council charged with overseeing matters of national security, packed it with loyal military officers and said it could operate only by majority vote.

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TimesCast | Egyptian Election Analysis

In Tahrir Square, celebrations began Monday after Egyptians elected an Islamist in their first competitive presidential election.

Officials of the Brotherhood, meanwhile, said the group was trying to rebuild its alliances with street activists and other civilian political factions for the coming conflict against the military. Senior officials of the Brotherhood’s party were meeting to invite others into a presidential advisory council and a cabinet to be formed under Mr. Morsi.

The Parliament-picked constitutional assembly, including representatives of factions across the spectrum, began meeting Monday night in defiance of the generals. In an apparent effort to head off legal challenges, it elected the head of Egypt’s judiciary, Hussam El Ghuriany, as its chief. He urged the swift drafting of a charter that protected minority rights, the Brotherhood said in a statement.

The Brotherhood’s leaders also said they would call out their supporters for a major demonstration on Tuesday against the reassertion of military rule. Brotherhood lawmakers said they would seek to enter Parliament despite the military guard and the general’s veiled threats. And the group pledged to join every such demonstration to follow, including one scheduled for June 30 — the day the generals have pledged a grand celebration for their nominal transfer of power.

“The revolution continues, and the political path can’t be victorious on its own,” Mohamed Beltagy, a Brotherhood lawmaker, wrote in an online appeal for other activists to join together as they did during the revolt against Mr. Mubarak. “When we were divided, the counterrevolution was able to gather its forces.”

The presidential election, said Emad Shahin, an Egyptian political scientist at the University of Notre Dame, had turned out not to be the promised capstone of the revolution as much as the beginning of a new stage of combat against the Supreme Council of the Armed Forces. “Pro-democracy forces have to wage yet another ferocious battle against the militarization of the political process,” he said, “and the insistence of the SCAF to place itself as a sovereign entity above popularly elected institutions.”

Mayy El Sheikh contributed reporting.

A version of this article appears in print on June 19, 2012, on Page A4 of the New York edition with the headline: After Victory in Egypt, Islamists Seek to Challenge Military. Order Reprints|Today's Paper|Subscribe